Background: Transforming growth factor-beta 1 (TGF-B1) is a pleiotropic cytokine which controls multiple cellular functions including cell proliferation, differentiation, apoptosis, and extracellular matrix (ECM) synthesis. TGF-B1 is a potent inducer of ECM protein synthesis and accumulation, and plays a key role in the pathogenesis of progressive diseases as a central mediator of fibrogenesis in a variety of tissues, including the kidney. However, the precise mechanisms responsible for the pathogenesis of renal fibrosis and progression to end-stage renal failure remain incompletely understood. Our previous studies have focused on the p38 mitogen-activated protein kinase (MAPK), a major stress signal transducing pathway that is rapidly activated by TGF-B1 in renal cells. We have identified MKK3 as the immediate upstream MAPK kinase required for activation of p38 MAPK and stimulation of pro-a1(l) collagen by TGF-B1 in murine mesangial cells and tubular epithelial cells. Our hypothesis is that the MKK3-p38 alpha and p38 delta MAPK signal transduction pathway is the critical mediator of tissue injury response in which TGF-B1 signals ECM synthesis and accumulation leading to progressive renal fibrosis. This proposal will focus on examining the cellular and molecular mechanism of TGF-B1 signaling, and we will further investigate the upstream activators of MKK3-p38 MAPK signaling pathway for TGF-B1, and examine their functional role in injury responses in renal tubular epithelial cells in vitro. In vivo correlates will be sought in an experimental model of renal fibrosis. We will employ state-of-the art approaches including a variety of dominant negative mutants of TGF-B receptors, the MAPKs and specific p38 isoforms, gene silencing by the use of RNAi (RNA interference) induced by short interfering RNA (siRNA), and genetically altered mice, the null mice for the various MAPKs, particularly the MKK3. Relevance: Although the central role of TGF-B1 in the development of renal fibrosis is well documented, general strategies to indiscriminately inhibit TGF-B1 actions altogether may prove to be imprudent. The studies in this proposal will yield important and novel information in furthering our understanding of the molecular mechanisms of TGF-B1 signal transduction, that we may be able to selectively block the pathway that signals the deleterious effects of TGF-B1.